Method of making nitrocellulose



Patented Aug. 16, 1932 UNITED STATES PATENT OFFICE NORMAN PICTON, OF STEVENSTON, SCOTLAND, ASSIGNOR TO E. I. DU PONT DE NEMOURS & COMPANY, OF WILMINGTON, DELAWARE, A CORPORATION OF DELAWARE METHOD OF LIAK ING NITBOCELLULOSE No Drawing. Application filed Kay 21, 1930, Serial No. 454,519, and in Great Britain May 27, 1929.

This invention relates to the preparation of nitrocellulose from pulp board.

Wood pulp appears in commerce in sheets resembling cardboard or thick paper and is of varying degrees of hardness and stiffness. This material, hereinafter referred to as'pulp board, may be in the form of flat boards, sheets or rolls. The usual method of nitration adopted has been to tear these boards up into pieces and nitrate the pieces with ordinary nitrating acid by the old pot method. The nitrating mixture as used in this or other known processes is low in nitric acid content and as an example of such a mixture, I mention one which contains roughly 22% nitric acid, 57% sulphuric acid and 21% water. \Vhile a nitrocellulose of a reasonably high nitrogen content can be obtained'by a mixture of this type, this nitrogen content is merely an average of an ununiform product, the nitration being irregular. The nitration of the center of the pieces of pulp is ve incomplete and this is shown by the Zict that such a nitrated product contains considerable amounts of material insoluble in acetone.

Another method of nitration is to pass the pulp through a Hollander and convert it into thin crepe paper which can be nitrated satisfactorily. This method, however, is ver expensive and the cost of production 0 the paper, owing to the low output from the machine, is high. 7

Another method which has been adopted is by disintegration of the wood pulp into its fibrous condition by passage through a disintegrating machine. A product is thus obtained Wl'llCh resembles to some extent short fibre linters and nitrates fairly successfully. These are certain disadvantages, however, in that the cost of disintegration is fairly high and the disintegrated material is bulky and retains a considerably larger proportion of acid after centrifugalling than does cotton linters. Moreover its bulky nature carries with it certain disadvantages in all stages of manufacture and in connection with its use in the formation of various final products.

This invention has as an object an improved method for the production of nitrocellulose from pulp board. A furthen object is the production of a nitrocellulose from pulp board of uniform quality which is capable of dissolving to a clear solution in the usual solvents and which is in a form having a high apparent density.

These objects are accomplished by the following invention in which pulp board is subdivided without disintegration into small pieces or tablets which are nitrated with a composition high enough in nitric acid to insure complete nitration.

I have now found that it is possible to nitrate wood pulp without previous disintegration so completely that the product dissolves to a clear solution in the usual solvents and is suitable for the technical uses which have been referred to above. In carrying the invention into effect in one form by way of example I systematically subdivide the sheet wood pulp into small pieces or grains as by a machine commonly used for cutting sheets of nitrocellulose plastics into powder'grains. Conveniently for wood cellulose sheet the pieces may be about A of one inch square giving a bulk density of about 340 gms. per litre in the case of the harder sheets of wood pulp and about 240 gms. per litre in the case of the softer and more easily disintegrated wood pulp'sheets. (These contrast very favorably with the bulk density of well pressed down cotton linters which is of the order of 80 gms. per litre.)

I effect nitration by the use of a nitrating acid composition containing a high percentage of nitric acid.

To produce a nitrocellulose of 11.7% nitrogen content I use an acid of the following composition Per cent HNO 70.0 H2 S04 20- 5 Dilution 9.5

The mixed acid illustrated above is suitable for the nitration of most grades of pulp board, but for board of maximum density mixed acid containing as high as 80% HNO may be required to avoid gelatinization. With lower density pulp board, a lower concentration of nitric acid may be used with satisfactory results and is to be preferred for economic reasons. In general the nitric acid content of the mixed acid is adi justed according to the character of the 5 pulp board within the range of from about 40 to about 80% HNO and "is, in all cases high enough to avoid gelatinization of the fiber, but preferably does not greatly exceed this proportion. As is well known in the nitrating art, the nitrogen content of the nitrocellulose is controlled by adjustment of the sulfuric acid to water ratio. Thus, I may use concentrations of nitric other than 70% and obtain nitrocellulose of about 11.7% nitrogen, while keeping the sulfuric acid to water ratio substantially as in the above example. On the other hand, I may produce nitrocellulose of other nitrogen content while adjusting the sulfuric to water ratio according to the usual practice to give the desired nitrogen content of the nitrocellulose, and at the same time adjusting the nitric content of the mixed acid having regard to the character of the pulp board used,

so as to be high enough but without unnecsary excess to avoid gelatinization during nitration.

The method of nitration may conveniently be the old pot process in which the cellulose is just dropped into the acid and allowed to stand for a certain time until nitration is judged to be complete. The acid is then centrifuged oil and the product drowned in excess of-.water, after which it may be stabilized in the usual manner and if necessary subjected to further treatment for the purpose of lowering its viscosity and stabilized, or it may be pulped before or after these treat- .ments.

The cellulose may also be nitrated by the displacement process in which case the sheets of pulp have to be cut up into pieces of such asize as to allow the ready displacement of the acid. It may also be nitrated by the ordinary mechanical process in which case the sheets have to be cut up into pieces of a size which are manageable in such an apparatus. I may also use a combination of both methods. The nitration may take place in some form of mechanical apparatus and the nitrated product be run out into displacement pans in order that the acid may be recovered by displacement. The high content of nitric acid increases the cost of the mixed acid and it is therefore desirable to recover as high a proportion of this as possible. By the displacement process a higher quantity of the mixed acid can be recovered and in our process two processes may be combined in order to recover a greater proportion of acid. 7

It is desirable for economic reasons to keep the proportion of nitric acid as low as is consistent with attaining the, result, avoiding on the one hand the tendency with lower nitric acid content to produce parchmentisation in the subsurface layers and consequent reduced solubility of the product, and on the other hand waste of the more expensive acid.

In all these methods of nitration the volume of acid in relation to the weight of cellulose can be very considerably reduced. Instead of employing proportions of acid varying from thirty to sixty times the Weight of cellulose we may employ as low as eight or indeed, in the case of the displacement process,.as low as five times the weight of the cellulose. In this way the output of the plant may be increased and the quantity of acid in circulation considerably reduced. Furthermore, if the product is finished in this dense condition it may be packed and transported far more economically than the fibrous material. This also is advantageous to the consumer because it is more easily handled and in addition a far greater weight can be stored in the same storage accommodation.

The invention is not limited tofinishing the material in this dense condition. As mentioned above it may be pulped at any stage after nitration and finished in the usual way. One eifect of the use of small quantities of acid is that the heat of nitration is now spread over a much smaller weight of material than is usually the case and we may employ cooled acids or use cooling means during the nitration process.

The proportions of nitric acid that should be used will vary somewhat with the class of wood or other raw material that the pulp is produced from. Certain of the hard, stiff wood pulp sheets require as much as 80% nitric acid in the mixed acid, while with the soft wood pulps the nitric acid may be re-. duced even to the order of 40%, but the nitric acid content of the mixed acid must be such that there is suflicient nitric acid present to insure penetration of a suitable composition of acid tothe inner layers of the cut sheet v and to insure complete nitration. I

The invention is not limited to the size to which the sheets of wood pulp may be cut. This size may vary from the smallest grains that can be cut, for instance, by such means as are comparable with those machines employed for cutting sheets of nitrocellulose plastics into powder grains. Such grains may also be readily produced by passing sheets of cellulose through a paper shaving machine to form narrow strips and cutting these into grains by subsequent treatment with the guillotine or they may be cut into small grains by means of rippers attached to the machines for producing the strips.

The width of the strips produced can be varied in such machines within wide limits.

Such small grains have in addition the dis tinct advantage to the user that they dissolve in solvents or solvent mixtures more easily and more quickly as they tend to keep separate during gelatinization and do not form lumps of gelatinized material which dissolve only very slowly. For still larger pieces, the sheets may be cut by ordinary guillotine machines. Another form may be as small circles punched out as in the case of confetti. In this way we may produce nitrocellulose in various forms all of which may be useful for the particular requirements of the consumer and all of which may be far more easily handled than the ordinary fibrous material.

It will be apparent that the invention is not limited to pulp board made from wood pulp but that the pulp board may be made with pulp obtained from bagasse, or from cotton cellulose, or from any other suitable source of cellulose pulp.

The nitrocellulose produced in accordance with the present invention yields, as far as possible, when dissolved in the various usual solvents, a clear solution, free from fiber and without color. These qualities render the product especially valuable in the manufacture of celluloid films and varnishes. For

the production of leather cloth, especially that of a dark shade, and of linoleum plastics, the above noted qualities need not be so high.

As previously noted the higher nitric acid content of the nitrating mixture is economical because the.volume of the acid with respect to the weight of cellulose is reduced. As further off-settingthe cost of the higher percentage nitric acid used in the practice of the present invention is the fact that the expense of disintegrating the pulp board is avoided. There is aconsiderable quantity of short fibres formed during disintegration and these short fibres tend to escape during centrifuging. The mixed acid high in nitric acid content results in quick penetration and fast nitration without the consequent gelatinization which occurs during the longer contact period of the cellulose with the higher percentage of sulphuric acid in the usual nitrating mixture containing a low percentage of nitric acid.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. A process of manufacturing nitrocellulose of substantially complete solubility which comprises granulatingv Pulp board without disintegrating and nitrating the granulated pulp board with a mixed acid composition containing from 40-80% nitric acid together with sulfuric acid and water, the nitric acid content being adjusted within this range so as to be above the concentration at which substantial gelatinization of the pulp boards occurs.

tinization of the pulp board occurs, said pulp board having a range of density such that when subdivided without shredding into pieces of about square the bulk density is not less than about 240 grams and not more than about 360 grams per liter. I

3. A process of manufacturing nitrocellulose which comprises subdividing pulp board without disintegration and nitrating the subdivided pulp board in a nitrating mixture of substantially the following composition:

Percent Nitric acid 70.0 Sulphuric acid 20.5 Dilution 9.5

4. A process of manufacturing nitrocellulose which comprises subdividing pulp board into small portions having the thickness of the pulp board and nitrating the subdivided pulp board in a nitrating mixture of substantially the following composition:

. Percent Nitric acid 70.0 Sulphuric acid 20.5 Dilution 9.5

In testimony whereof, I aflix my signature.

NORMAN PICTON. 

